This invention occurs in the field of art involving, generally, lubricating oils adapted for use between a plurality of relatively moving surfaces with which the fluid compositions are in contact for the purpose of reducing the friction between these surfaces while providing protection, concomitantly, from wear and corrosion. These fluid compositions or lubricating oils, tend to deteriorate under conditions of use in present day diesel engines with attendant formation of sludge, lacquer and resinous materials which adhere to the engine parts; particularly the piston ring, groove and skirt, thus reducing the operating efficiency of the engine. To counteract the formation of these deposits, certain chemical additives have been found which when added to lubricating oils have the ability to keep the deposit forming materials suspended in oil so that the engine is kept clean and in efficient operating condition for extended periods of time. These agents are known in the art to which this invention pertains as detergents, dispersants or detergent-dispersants. Metal organic compounds are particularly useful in this respect. One class of metal organic compounds which has been found so useful are the sulfurized normal and overbased calcium alkylphenolates. These agents are believed to be effective because they provide alkalinity capable of neutralizing strong organic and inorganic acids and are capable of dispersing deposits and deposit precursors into the oil phase. Overbased sulfurized metal alkylphenolates have been found to be particularly effective dispersants in lubricating oils.
By the term "overbased" in this context is meant, generally, that the ratio of the number of equivalents of calcium metal to the number of equivalents of alkylphenolate moiety is greater than 1. The term "normal" indicates that the ratio of the number of equivalents of calcium metal to the number of equivalents of alkylphenolate moiety is 1. U.S. Pat. Nos. 3,528,917; 3,549,534; 3,761,414; and 3,969,235 describe lubricating compositions containing sulfurized normal calcium alkylphenolate detergent-inhibitors of improved resistance to oxidation. Sulfurized overbased calcium alkylphenolate additives of particular utility are those described in U.S. Pat. Nos. 3,474,935 and 3,706,632. The term "overbased" in the present instance is further defined to mean that the ratio of the number of calcium moiety to the number of equivalents of alkylphenolate moiety is at least 2.9:2; that is, a calcium metal to alkylphenolate ratio of at least 2.9:2. In contrast, many overbased sulfurized metal alkylphenolates having a calcium metal to alkylphenolate ratio greater than 1 and less than 2.9 have also proved to be useful lubricant additives heretofore.
Other useful detergent-inhibitor additives are the alkaline earth metal sulfonates, and preferably, calcium hydrocarbyl sulfonates wherein the hydrocarbyl sulfonate moiety is derived from a petroleum sulfonate.
It has been found recently that railway diesel engine oils having a high degree of alkalinity, that is, a TBN of at least 10 are particularly desirable in that they prevent corrosion by oil-soluble acids formed by oxidative deterioration at the high temperature existing under normal conditions of engine employment in proximity to the combustion chamber. The term "TBN" or "nominal "TBN" as employed herein refers to "total base number" which is defined as the quantity of acid, expressed in terms of the equivalent number of milligrams of potassium hydroxide that is required to neutralize all basic constituents present in one gram of a given sample. The method of evaluation is that defined as ASTM Method D 664.
While the desired alkalinity can be attained by introduction into the lubricant oil of a calcium sulfonate such as the foregoing having a nominal TBN of 290 or more of a sulfurized overbased calcium alkylphenolate having a calcium metal to alkylphenolate ratio of at least 2.9:2, the resulting lubricant compositions are unsatisfactory because these overbased materials degrade the silver protection characteristics of the oil, a factor of particular significance with respect to railway diesel engines, the vast majority of which, in the United States, and to a significant extent outside of the United States, as well, utilize silver-plated piston pin insert bearings.
Accordingly, the discovery of additives which would permit the use of a highly overbased calcium alkylphenolate and, when desired, a highly overbased alkaline hydrocarbyl sulfonate, as well, in lubricant oils for use in railway diesel engines in amounts sufficient to provide a TBN of at least 10, without diminution of the silver properties of the oil, would represent a significant advance in the state of the art.